1. Field of the Invention
The present invention generally relates to light source driving units and optical storage apparatuses, and more particularly to a light source driving unit, such as a laser driving (or controlling) unit, which drives (or controls) a light source by a light modulating waveform having multi-values or multi-levels, and to an optical storage apparatus which uses such a light source driving unit.
The light source driving unit according to the present invention may be used in image forming apparatuses and information recording and/or reproducing apparatuses. The optical storage apparatus according to the present invention includes information recording and/or reproducing apparatuses such as MD drives, MO drives, CD-R drives, CD-RW drives, DVD-R drives, DVD-RW drives, DVD+RW drives and DVD-RAM drives.
2. Description of the Related Art
In an optical disk drive which carries out a recording by modulating light, techniques for controlling a light modulating waveform which drives a light source to have multi-values or multi-levels are essential, in order to realize a 1-beam overwrite or to control a shape of a recording mark for increasing the recording density on an optical disk. Hence, in a light source driving unit (hereinafter also referred to as a laser diode driver or simply LD driver), it is necessary to switch a plurality of laser diode driving currents, and a number of input signal lines increases depending on the number of laser diode driving currents.
Because the demands to further improve the high-speed recording and high-density recording with respect to the information recording medium are increasing, and further increase in a data transfer rate, further narrowing of a pulse dividing width, and further increase in a number of power levels are unavoidable in the future.
An optical pickup which irradiates a laser beam on the optical disk is movable in a radial direction of the optical disk to carry out a so-called seek operation. Hence, in general, the optical pickup and a circuit board which is mounted with a signal processor and the like are connected via a flexible printed circuit (FPC). An LD driver is disposed in a vicinity of a light source (laser diode: LD) which is provided on the optical pickup. The signal processor and the like on the circuit board and the LD driver on the optical pickup are thus connected via the FPC.
However, it is inevitable that the FPC must have a certain length in order to allow movement of the optical pickup. Hence, light modulation control signals which are supplied to the LD driver via the FPC are subjected to waveform distortions and delays caused by signal lines of the FPC. As a result, an error is introduced in switching timings of the LD driving currents due to the waveform distortions and delays of the light modulation control signals, and a waveform distortion is generated in the LD driving current at a timing when switches for switching the LD driving currents are switched simultaneously. This waveform distortion of the LD driving current makes it difficult to emit the laser beam with a desired light waveform. Of the delays of the light modulation control signals, a difference in the delays of the plurality of light modulation control signals is often referred to as a skew.
FIG. 1 is a system block diagram showing an example of a conventional LD driver, and FIG. 2 is a timing chart for explaining the operation of the conventional LD driver.
In FIG. 1, a current source section 300 supplies currents Ib, Ie and Iw respectively corresponding to light irradiation levels of a laser diode (LD) 303 which is used as a light source. A switching section 301 includes switches SW1 and SW2 which are respectively switched in response to control signals S1 and S2. An adding circuit 302 adds the currents Ib, Ie and Iw which are selectively output via the switching section 301, and outputs a LD driving current for driving the laser diode 303. When the control signal S1 has a high level and the control signal S2 has a low level, a current Ib+Ie is supplied to the laser diode 303 to emit light with an erase power Pe. When the control signal S1 has a low level and the control signal S2 has a high level, a current Ib+Iw is supplied to the laser diode 303 to emit light with a write power Pw.
However, if a delay is generated in the control signal S1 as indicated by m in FIG. 2 and a skew is generated between the control signals S1 and S2, a waveform distortion is generated in-the light waveform when the power of the light emitted from the laser diode 303 changes from the erase power Pe to the write power Pw, as indicated by a portion surrounded by dotted lines in FIG. 2.
When the laser diode 303 cannot emit the light (laser beam) with the desired light waveform, the accuracy of the mark shape and the mark position on the optical disk deteriorates, to thereby cause data error. The effects of the waveform distortion in the light waveform is particularly notable when carrying out a high-speed recording with respect to the skew peculiar to the optical disk drive.
For example, if a skew of approximately 1 ns is generated in the optical disk drive, 1 channel clock period T is approximately 230 ns when carrying out a CD 1-times speed recording. Hence, a pulse width must normally be set with a resolving power of approximately T/32 (approximately 7 ns) with respect to the channel clock period T. In this case, the skew of approximately 1 ns does not generate serious problems and is tolerable. Of course, a resolving power of approximately T/40 may be required depending on the optical disk used.
But if a CD 48-times speed recording is to be carried out, the channel clock period T is approximately 4.8 ns, and the pulse width must be set with a resolving power of approximately 150 ps. In this case, the skew of 1 ns is not tolerable, and if such a skew is generated, the laser diode 303 cannot emit the light (laser beam) with the desired light waveform, the accuracy of the mark shape and the mark position on the optical disk deteriorates, to thereby cause the data error.
Furthermore, radiation from the FPC causes noise to be generated in the signals.
For example, a light source driving unit was proposed in a Japanese Laid-Open Patent Application No. 11-283249 to solve this problem. The proposed light source driving unit includes a laser diode driving means for supplying currents from a plurality of current sources to a laser diode via a switching means, and a driving waveform restoration means for restoring a driving waveform.(light modulating waveform) which drives the laser diode in correspondence with a binarized recording signal to be recorded on the information recording medium and controls the switching means. The laser diode driving means and the driving waveform restoration means are provided on a single laser driving integrated circuit, so as to prevent the generation of skew by reducing the length of the wirings between the laser diode driving means and the driving waveform restoration means.
However, even when the laser diode driving means and the driving waveform restoration means are provided on the same integrated circuit, it is extremely difficult to make the delays of switches, the delays of circuits which generate switching control signals, the lengths of control signal lines, the load conditions and the like identical with respect to all of the light modulating control signals, and the skew is inevitably generated. Hence, when further improvements made in the high-speed recording, even a slight skew will not be tolerated, and the simple reduction of the skew will not solve the above described problems for the super high-speed recording.
When the high-speed recording and the high-density recording with respect to the information recording medium are further improved, a light modulating control signal generator (driving waveform restoration means) will need to operate at a higher operation speed and a higher integration density will be required. An extremely fine CMOS process is suitable for the purpose when realizing such high-speed-operation and high integration density of the light modulating control signal generator. But on the other hand, the laser diode driver is connected to the laser diode which has an operating voltage of approximately 1 V to several V, and a high withstand voltage process (for example, 5 V or 3.3 V) is required.
However, it is normally difficult to realize a high withstand voltage in the case of the extremely fine CMOS process, because the withstand voltage is only approximately 1.8 V in the case of a CMOS process of 0.18 μm. As a result, there are problems in that it is difficult to realize the high-speed operation of the light modulating control signal generator, the cost of the LD driver considerably increases, the power consumption of the LD driver increases, and the size of the integrated circuit as a whole increases.
Furthermore, complex light modulating waveforms are required depending on the information recording media. For example, when carrying out a high-speed-recording, the passing time of the irradiated light beam on the information recording medium becomes short, and the amount of energy irradiated on the information recording medium decreases, and the amount of heat generated may become smaller than that required to form the recording marks on the information recording medium. Hence, in order to accurately carry out the recording, the recording should be made using a pulse train having an extremely narrow pulse width, but such a narrow pulse width would require a high laser power of the light source. Accordingly, there is a proposed method which carries out the recording at a relatively low laser power by decreasing the frequency of the multi-pulse train (or sequence).
On the other hand, when the recording is carried out at a low speed with respect to the information recording medium which has an improved recording sensitivity for use in the high-speed recording, the heat generated may become excessively large to make it impossible to accurately form the recording marks on the information recording medium. Hence, there is a proposed method which carries out the recording by increasing the frequency of the multi-pulse train.
Therefore, various recording methods have been proposed for the various kinds of information recording media, but none of the proposed methods can cope with the various kinds of information recording media using the same circuit. In order to cope with the various kinds of information recording media, various light modulating waveforms are required by changing the frequency of the multi-pulse train and providing multi-levels.